The invention relates to a data transmission system comprising a transmitter and a receiver coupled thereto via a transmission path, the transmitter comprising a transversal smearing filter and the receiver comprising a transversal desmearing filter, the transversal filters each comprising a plurality of series-arranged delay elements, the time delay .tau. of each element being of the same duration as the sampling period of an input signal, and a signal processing arrangement coupled to taps arranged between every two consecutive elements, for, during at least each symbol interval T, multiplying the signals present on the taps by individual coefficients determined for each tap, and summing the product signals thus obtained, the signal processing arrangement also being coupled to an input of the first element and to an output of the last element.
Such a data transmission system is disclosed in inter alia U.S. Pat. No. 4,285,045, to Tamori et al.
For data transmission, use is often made of the public telephone network. Such a network introduces a number of imperfections such as: amplitude and phase distortions, frequency offset, phase jitter and both additive and impulsive noise. The effects of most of these imperfections are reduced or eliminated in modulation band systems with the aid of rather sophisticated digital modems. However, until now, little attention has been paid in the design of modems to reduce errors due to impulsive noise. The effects of impulsive noise on the transmission, affect predominantly switched connections and become the more noticeable as the transmission rates become higher.
A solution to combat the influence of impulsive noise is in the use of a smearing filter at the source side of the transmission system and a desmearing filter at the receiver side of the transmission system. In general, these filters have a flat amplitude response with a group delay time which linearly increases or decreases, respectively, as a function of the frequency, the sum of the group delay times of both filters being constant to the best possible extent. That is to say that the group delay time of one filter is complementary to the group delay time of the other filter. A data signal which passes through both filters is consequently only delayed in the case of ideal filters. However, a noise pulse passes only the desmearing filter so that the energy of such a noise pulse is smeared in time, so that its effect on the data signal is considerably reduced at any instant.
An analog implementation of such filters is described for modulation band systems in the article "On the potential advantage of a smearing-desmearing filter technique in overcoming impulse-noise problems in data systems", (by R. A. Wainright), published I.R.E. Transactions on Communication Systems, December 1961. In view of the stringent requirement that the group delays of both filters must be accurately matched, such an implementation is, in practice, rather complicated, more specifically because of the variation of the filter characteristics. To that end, it is known from U.S. Pat. No. 4,285,045 to use, for baseband signals, digital implementations of these filters, more specifically in the transversal form.
In order to obtain a proper smearing of the pulses, comparatively long filters are, however, required. The number of multiplications required per output signal for the weighting procedure is generally equal to the length of such filters. Moreover, if these filters are realized in the digital form, the coefficients for such filters have a word length of 8 bits. The associated large number of multiplications complicates the construction of such filters.